Increased environmental concerns and regulations over the use of volatile organic compounds (VOCs) since the late 1980s (e.g. Montreal Protocol in 1987 and the Clean Air Act amendments in 1990) have caused considerable effort being put into finding environmentally benign solvents for industrial use (McHugh, M. A. and V. J. Krukonis, Supercritical Fluid Extraction: Principles and Practice. Second ed, ed. H. Brenner. 1994, Boston: Butterworth-Heinemann). DeSimone et al. at the University of North Carolina-Chapel Hill have shown that supercritical carbon dioxide (scCO2) is a viable and promising alternative solvent (Tc=31.8° C., Pc=76 bar) to perform free-radical, cationic and step-growth polymerizations using batch reactors (DeSimone, J. M., Z. Guan, and C.S. Elsbernd, Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science, 1992. 257: p. 945-947). This work has been summarized in several recent reviews (Kendall, J. L., et al., Polymerizations in Supercritical Carbon Dioxide. Chem.Rev., 1999. 99(2): p. 543-563; Canelas, D. A. and J. M. DeSimone, Advs. Polym. Sci., 1997. 133: p. 103-140; Shaffer, K. A. and J. M. DeSimone, Chain Polymerizations in Inert Near and Supercritical Fluids. Trends in Polymer Science, 1995. 3(5): p. 146-153). Indeed, CO2 technology is intended to be commercially implemented by 2006 for the manufacture of Teflon™ by DuPont (McCoy, M., DuPont, UNC R&D effort yields results, in Chemical & Engineering News. 1999. p. 10). The reasons for the intense industrial interest are that CO2 is cheap ($100-200/ton), of low toxicity, non-flammable, and environmentally and chemically benign. In comparison to existing technologies for making polymers, CO2 technology has several significant advantages as it will allow for the elimination of: a) expensive polymer drying steps; (b) expensive wastewater treatment and disposal steps where significant amounts of monomer, surfactants and emulsifiers are generated (Baker, R. T. and W. Tumas, Toward Greener Chemistry. Science, 1999. 284: p. 1477-1478); (c) disposal of “spent” organic solvents; (d) handling, storage and shipping of toxic organic solvent; and (e) chain transfer to solvent, i.e., a reaction that may limit the achievable molecular weight of the polymer.
As industrial interest in using scCO2 as a polymerization medium has grown, several disadvantages of batch reactors have been recognized, including: (1) large reactors which are costly at the high pressures of scCO2; and (2) difficulty in recycling the CO2 and the unreacted monomer. Accordingly, there is a need for new ways to carry out the continuous polymerization of monomers in carbon dioxide, particularly liquid and supercritical carbon dioxide.